At present, iliac bones, fibula bones, and the like of a patient are often used for regenerating bone defects. Use of a patient's own bones, however, imposes a heavy burden on a patient, and the amounts thereof that can be obtained are limited. Accordingly, it is necessary to supplement bone defects with artificial bones or artificial fillers. Properties required for such artificial biomaterials are mechanical properties such as strength and elasticity similar to those of natural bones and biological properties such as bioadaptability and bone conductivity.
Bones of vertebrates are originally composed of an inorganic substance (hydroxyapatite (HAp)) and an organic substance (collagen (Col)). They forms a specific nanocomposite structure in natural bones characterized in that the c-axis of hydroxyapatite is oriented along collagen fibers (self-organization), and this structure imparts bone-specific mechanical properties. Specifically, a simple combination of HAp and collagen cannot provide structures or properties similar to those of natural bones.
In addition to biocompatibility, it is necessary for such artificial biomaterials to have effects of actively accelerating bone regeneration by being fused with bone tissues. Specifically, artificial materials need to have bone conductivity or bioactivity so that the materials can being gradually resorbed after implantation in the body, and involved in the bone regeneration cycle, thereby substituting the patient's own bone. An inorganic substance (HAp) has excellent bone-compatibility, and an organic substance (collagen) is capable of accelerating cell adhesion and cell differentiation. Accordingly, composites of these two substances are expected to have excellent properties as artificial biomaterials.
A variety of studies have been made to develop organic/inorganic composite biomaterials that are more similar to natural bones by using hydroxyapatite and collagen. For example, JP Patent Publication (Kokai) No. 7-101708 A (1995) discloses a process for producing an apatite-organic substance composite in which a molded body having the Young's modulus similar to that of natural bones is obtained with the gradual addition of a mixed solution of collagen and phosphoric acid in a suspension of calcium hydroxide. JP Patent Publication (Kokai) No. 11-199209 A (1999) discloses a process for producing organic/inorganic oriented composite materials, wherein an aqueous phosphoric acid solution containing collagen and an aqueous calcium salt solution are simultaneously added dropwise to a reaction vessel while regulating pH and temperature at the time of reaction, and a resulting sediment is pressed, thereby obtaining materials similar to natural bones. Further, JP Patent Publication (Kokai) No. 2000-5298 A discloses a technique for accelerating the formation of apatite on a collagen surface with the use of organic acid.
Fiber lengths of composites obtained by such conventional techniques were, however, approximately between several μm and 20 μm, which was not yet sufficient to realize self-organization.